A conventional power amplifier (PA) in a digital wireless transceiver receives and then amplifies a radio frequency (RF) signal. For many modulation schemes, including those that comply with the IS-95, PDC, PHS, DCT and certain CDMA standards, the RF signal has a non-constant envelope, which requires the power amplifier to operate with a high degree of linearity over a large dynamic range. As a result, the conventional power amplifier usually is biased for operation between the Class A and Class B modes. The best case (asymptotic) efficiency of a PA operating in this manner usually lies between 50% and 78% for constant envelope signals; the efficiency is even lower for non-constant envelope signals.
U.S. patent application Ser. No. 09/108,628, filed on Jul. 1, 1998, by Donald Brian Eidson and Robert Edmund Grange, and titled "Envelope Feedforward Technique with Power Control for Efficient Linear RF Power Amplification," discloses a power amplification technique that allows operation in high efficiency modes (such as Class D or Class E), in some cases producing asymptotic efficiencies for constant envelope signals that approach 100%. In one implementation of this technique, a dual-gate field effect transistor (FET) acts in a manner similar to a conventional RF mixer, receiving at one dual-gate input a constant envelope RF signal containing phase component information, and receiving at the other dual-gate input an unmodulated signal containing the envelope component. In a digital system, the constant envelope RF signal and the unmodulated envelope signal are derived from the in-phase (I) and quadrature (Q) components provided by a baseband device.